I just boot the Ubuntu OS 16.04.1, open the terminal and enter the commands.
Then an error message was shown:
"Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?"

I wait for a while, and run the command again, now the message was changed:
"Reading package lists.... Done
E: You must put some 'source' URIs in your sources.list"

But from the forum it seems that everyone is using it correctly.
I install 14.04.5 build and finally the command is working normally.
So, my question is, which server I should add to the sources.list in 16.04?
And then I can use it in the latest Ubuntu?

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